Most subscribers served by multiple telephone lines find that the basic features that key telephone systems generally provide are quite desirable. These features consist of (1) visual signaling for indicating which of the lines is being rung, is in use, or on hold; (2) selective pickup for enabling the use of any of the lines from a single set; and (3) selective application of a hold bridge for allowing a subscriber busy on one line to hold the connection to that line and place or receive a call on another line. It is clear that these features greatly facilitate efficient use of multiple telephone lines, and this has resulted in widespread use of key telephone systems.
However, many small business subscribers whose needs are satisfied by four telephone lines or less serving 20 stations or less find that the key telephone systems presently available cost more than they wish to spend and/or require more space than they are able or willing to give up. The problem to be solved therefore is the design of a key telephone system for such a small business environment that is less expensive than existing equipment and occupies a small amount of space.
My solution to this problem relies upon the use of electronic digital circuitry. The 60 Hertz power required to operate key telephone systems provides the basic timing function needed for such circuitry. In addition, such circuitry lends itself to integration which provides economies of both space and cost.
The use of digital circuitry in a key telephone system is disclosed in U.S. Pat. No. 3,604,857 issued to D. C. Opfermann, on Sept. 14, 1971. However, in that key telephone system, in addition to a line circuit or module being associated with each telephone line, a station module is associated with each station set, interconnection therebetween being accomplished by crosspoint modules. Furthermore, each station set must transmit and receive digital data signals and therefore special purpose rather that standard station sets must be used.
Finally, in that key telephone system, clock pulses are used to establish time slots, each of which is assigned a particular bit of information. The bits of information are transmitted in a particular sequence between each line module and the associated station modules and between each station module and its associated station set. In addition each line module is enabled in a particular sequence.
Each line module includes a 9 state counter and when a particular line module is enabled in its turn, the counter thereof is always stepped through all 9 states. The first 5 states serve to transmit information bits to the associated station modules, 4 of the 5 states causing enabling pulses to be applied to individual transmit logic gates that provide unique outputs dependent upon information bits received from the associated station modules during the previous advancement of the counter. The sixth state causes an enabling pulse to be applied to the associated crosspoint modules, while the last 3 states serve to receive information bits from the associated station modules, these 3 states causing enabling pulses to be applied to individual receive logic gates that provide unique outputs dependent upon information bits concurrently received from the associated station modules. The receive logic gates serve to set or reset memory flip-flops that provide inputs to the transmit logic gates during the next advancement of the counter.